In the manufacture of laminated substrates, such as printed circuit boards, flux is used to hold surface-mounted components in place until the components can be soldered permanently to the substrate. Flux acts as an adhesive to hold the components in place pending soldering. Flux also minimizes metallic oxidation which can occur at soldering temperatures. The result is improved reliability of the electrical connection.
Flux is a suspension liquid typically comprised of various low-solids acid suspended in an alcohol base. Flux is applied to the areas on the substrate where a component will be soldered (the "component placement sites"). Controlling the volume of flux applied to the component placement sites is important, since enough flux must be used to cover a given site; but too much flux is undesirable, since it impacts other circuit board manufacturing processes. For example, flux is used particularly to hold C4 chips in place during the soldering of the chips to the substrate. If too much flux is used with C4 chips, the flux residue adversely impacts chip underfill reliability.
Placement of the components onto the substrate must be performed while the flux is wet, or the flux will not be tacky enough to hold the components in location prior to soldering. However, the no-clean flux developed for this application dries very quickly (in seconds). Therefore, fluxing must be performed immediately before placement of the components. As a result, the fluxing process must be a part of the automated placement of the components.
Flux has been applied in the past using a variety of manual and automated methods. Popular methods include dipping, brushing, syringe dispensing, spraying, or atomizing flux onto the component and/or substrate prior to placement of the component on the substrate. Transfer methods of applying flux also may be used. A transfer method involves dipping a compliant tip into a flux bath, moving the tip over to a component placement site, and pressing the tip onto the site to deposit flux at the site.